The present invention relates in general to ultrasonic inspection and, more particularly, to the use of ultrasonic transducers to detect and size flaws in composite or other laminated structures.
The use of composites for aircraft structures has experienced a significant growth in recent years. Such structures are typically formed by curing or consolidating multiple layers of graphite/epoxy or other fibers resin composite materials into a laminate of desired configuration. As fabrication techniques have improved, it has become possible to produce large-scale composite panels having stiffeners integrally molded thereto. These stiffeners have been variously shaped, ranging from simple plate-like blades to angled structures having a T, I, L, or Z cross-sectional configuration.
Heretofore, composite structures have been principally inspected with a hand-scanning method in which a hand-held yoke, carrying either through-transmission or pulse echo transducers, is manipulated by hand over the part under study. While the hand-scan methods have been successfully used to locate flaws in composite structures, they have not been entirely satisfactory. For example, since the technique is dependent upon the freehand guidance of the yoke by the operator, the technique is inherently slow and unreliable since there is no assurance that there will be 100% coverage of the part under inspection. In addition, since the operator must simultaneously manipulate the yoke and observe and analyze a displayed output of the ultrasonic transducers, it is difficult to accurately and reliably identify and locate flaws.
In addition to the foregoing disadvantages, these prior arrangements have failed to adequately and reliably provide information concerning flaws in the critical junction or radius regions where two composite elements intersect as, for example, where a stiffener is integrally joined to a skin panel. In these regions, layers of piles on opposite sides of the stiffener smoothly curve through a bend (typically 90 degrees) and then diverge away from one another where the stiffener meets the skin panel. As a result of this divergence, there exists a cavity of filler area of generally triangular cross-sectional shape having a flat base formed by the skin panel and two concave sides formed by the facing surfaces of the bent portions of the layers that make up the stiffener. In the production process, this filler area is completely filled in a variety of ways to provide a solid structure.
From an inspection standpoint, this filler area is critical since delamination occurring adjacent the bottom or sides of the filler region can result in a loss of up to approximately 80% of the strength of the part. A decrease in strength can also occur as a result of voids in the filler region itself. Despite these significant concerns, it has proved to be a difficult and complex task to locate flaws in the radius region of such composite parts. It is altogether a different and much more difficult task to properly size the flaw so that its impact on the strength of the part may be fully assessed. This difficulty occurs because of the differences in the orientation and actual construction of flaws in composite or other laminated materials relative to the flaws or voids that occur in homogeneous materials.
The flaws in composite or other laminate materials tend to follow the direction of the plies or laminates. In addition, the laminates themselves and their constituent fibers tend to act as waveguides so that there is little predictability concerning the direction that injected sound will follow. These difficulties are even more pronounced in the radius region, owing to the curvature of the laminates and the nature of the filler region, where flaws or voids of many different shapes and orientations are possible. As a consequence, prior inspection techniques that rely upon amplitude sensing are not well suited for inspecting laminated structures and, particularly, the critical radius regions thereof.
The present invention aims to overcome these disadvantages by providing a new and unique ultrasonic inspection apparatus that can be used to make a single-pass, automated inspection of laminated parts. A further and principal aim is to provide such an apparatus that can provide substantially 100% coverage of the part being inspected, including any radius and blade shadow regions.